The present invention relates generally to a magnetic resonance imaging (MRI) system, and more particularly to an open MRI magnet system having a vibration isolation system.
MRI magnets include resistive and superconductive MRI magnets used in various applications, such as medical diagnostics. Known superconductive MRI magnets include liquid-helium-cooled, cryocooler-cooled, and hybrid-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled MRI magnet typically also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in solid conduction thermal contact with the thermal shield, and having its second stage in solid conduction thermal contact with the superconductive main coil. A liquid-helium-cooled MRI magnet typically also includes a liquid-helium vessel surrounding the superconductive main coil with the thermal shield surrounding the liquid-helium vessel. A hybrid-cooled MRI magnet uses both liquid helium (or other liquid or gaseous cryogen) and a cryocooler coldhead, and includes designs wherein the first stage of the cryocooler coldhead is in solid conduction thermal contact with the thermal shield and wherein the second stage of the cryocooler coldhead penetrates the liquid-helium vessel to recondense “boiled-off” helium.
Known resistive and superconductive MRI magnet designs include closed MRI magnets and open MRI magnets. Closed MRI magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a magnetic field of high uniformity within a typically spherical imaging volume centered within the MRI magnet's bore where the object to be imaged is placed.
Open MRI magnets, including “C” shape and support-post MRI magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed MRI magnet design.
It is also known in open MRI magnet designs to place an iron pole piece in the bore of a resistive or superconductive coil assembly. The iron pole piece enhances the strength of the magnetic field and, by shaping the surface of the pole piece, magnetically shims the magnet improving the homogeneity of the magnetic field. Nonmagnetizable support posts are connected to the face of the pole pieces. It is additionally known in horizontally-aligned open MRI magnets to support the magnet on the floor using two spaced-apart feet attached to each assembly, such feet raising the assemblies to provide room underneath the assemblies for necessary wires, pipes, etc.
The sharpness of an MRI image depends, in part, on the magnetic field in the imaging volume being time-constant and highly uniform. However, the magnetic field in prior art systems suffers time and spatial deformation caused by vibrations from environmental disturbances. Minor relative motions between any of the magnetic elements will cause significant magnetic field disturbances, thus reducing the image quality.